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The placenta grows wherever the embryo implants itself in the uterus. If the embryo implants itself in the lower portion of the uterus, the placenta might grow over the cervix — causing placenta previa.

Most cases of placenta previa are diagnosed during a second trimester ultrasound examination. If the placenta just barely reaches the cervix, expansion of the uterus may pull the placental attachment higher, away from the cervix, which will resolve the situation.

If the placenta is across the cervix, however, it's unlikely to resolve with time. The later in pregnancy that placenta previa exists, the more likely it will be present at the time of delivery.

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